I am posting this to close out the thread and hopefully help someone else with these issues. Before breaking the carb apart I decided to change the spark plug which resolved the issue of the kart dying after a few laps. After changing the spark plug the apparent rich condition was also eliminated once the engine was brought up to temp (110 degrees). As a side note, I also added an additional breather tube to my gas tank cap using a 1/4 inch copper barb which eliminated a bogging issue I was having when I had more than a half tank of gas and I would come off the gas and try to get back on it.
To summarize, here’s the issues I had and the things that fixed them:
Bogging in top of 5th and all of 6th gear. Rebuilt Mikuni fuel pump which solved this issue (one of the two small internal diaphragms was bent).
Excessive white smokey exhaust, heavy bubbling in the radiator and overheating. Replaced head gasket which resolved all these issues.
Kart dying after 2 solid laps. Changed spark plug which resolved this issue.
Bogging when coming off the gas and then getting back onto it when the gas tank was half or more full. Added breather tube on gas cap which resolved this issue.